This invention relates generally to devices for locating flaws or defects in structural materials and pertains more particularly to electronic probes for measuring the progation of ultrasonic acoustic waves through structural materials.
A problem faced by manufacturers of structural materials is the need to provide materials which can be relied on to meet specified performance criteria such as strength and fatigue resistance. In order to insure that such criteria are met, a method of testing is often used. For very expensive structural materials and materials used for applications in which structural failure would be catastrophic, a non-destructive testing technique must be used in order to insure that each structure produced is sound. For example, expensive structural materials in the framework of an aircraft must be tested in order to insure that the aircraft does not come apart in flight.
It is important that non-destructive testing techniques be capable of evaluating the soundness of all sizes and shapes of structural materials in use. Typically, such testing is used to decide whether a structural material having a defect or flaw in it should be replaced. The testing may be performed at the time that the structural material is fabricated and may also be performed in the field periodically to locate defects which may develop with time.
Structural materials which are candidates for non-destructive testing may include metals such as aluminum, titanium, and stainless steel which may be electrically conductive or may be substantially non-conductive electrically. Structures composed of non-metals such as boron graphite which is electrically conductive may be candidates for non-destructive testing. Non-metallic, electrically non-conductive materials such as fiberglass or graphite composites are popular structural materials which are candidates for non-destructive testing. A popular composite structural material now in use includes aramid fibers.
Structural materials in common use are formed as relatively homogeneous solids, as laminated solids with cross-plys, and as honeycomb or corrugated structures. A particular objective in non-destructive testing is to locate voids or defects in adhesive bonding between layers inside laminated solids. Such laminated solid structural materials are fabricated by bonding together sheets of material, such as resin impregnated fiberglass, with adhesives. Voids or defects may result in such laminated structural materials where the adhesive has failed or where adhesion has been prevented by water or other contaminants.
A particular type of structural material which has recently become popular is a composite material composed of a matrix of aligned, elongated graphite fibers embedded in a resin and which is substantially non-conductive electrically. Such materials are commercially available under the trademarks "Rhordyne" (from Rohr Industries) and "Kevlar" (from DuPont). Parts made from structural materials with aligned graphite fibers are characterized by low mass, high strength, and strength maximized and concentrated in preselected directions and areas. Structural materials containing aligned graphite fibers are candidates for non-destructive testing to locate graphite fibers which are mis-aligned and to find defects where the graphite fiber matrix is distorted or irregular. The strength and performance of the structural materials containing aligned graphite fibers is very much dependent upon the uniformity and regularity of the fiber matrix since the function of the matrix is to insure that the fibers cooperate together to mutually reinforce the structural material.
Structural materials which are candidates for non-destructive testing are fabricated into parts used for fixed and rotary structures, control surfaces, fairings, fuselage skin bonded lap joints, rotary wing blades, root stiffener sections, anti-erosion or ablative coatings, and aircraft interior paneling and floor structures. Such parts often have elaborate and precisely convoluted shapes with narrow openings and curved surfaces. Parts which are very expensive or designed for functions requiring high reliability, are particularly candidates for non-destructive testing.
A prior method used in the non-destructive testing of electrically conductive structural materials involves the use of a magnetic field to induce eddy currents in the structural material to be tested. The induced eddy currents produce mechanical acoustical vibrations in the structural material, which, in accordance with the prior method, are detected by a microphone.
Another prior method used in the non-destructive testing of structural materials involves transmitting mechanical acoustical vibrations into the structural material at a narrow end or point. As above, the mechanical acoustic vibrations passing through the structural material are detected in this prior method by a microphone which may be coupled to the structural material through a receiver point contact spaced apart from the transmitter point. In this prior method, a known technique for reducing noise and crosstalk errors is to measure the second harmonic of the transmitted frequency.
A problem with the prior eddy current non-destructive testing method is that the method will not work with electrically non-conductive structural materials. A problem with the point contact method mentioned above is the lack of sensitivity to measure small defects or voids and to detect misaligned fibers or distortions in the fiber matrix of structural materials having aligned graphite fibers. Another problem with the point contact method is the orientational sensitivity of the method such that defects directly between the transmitter and receiver points may be detected but defects displaced from between the transmitter and receiver points may evade detection. In particular, such orientational sensitivity increases the likelihood that narrow, elongated defects will be overlooked. If the line between the transmitter and receiver points is not passed through the width of such a narrow, elongated defect, then the defect may well go undetected. Such orientational sensitivity gives rise to the requirement that a structural material be extensively scanned with the transmitter and receiver points in order to perform a complete test. A problem with both prior methods is that rather large probes are required which are not compatible with the intricate and demanding shapes of the structural materials to be tested.
In practice, testing with either the eddy current method or the point contact method involves scanning a probe over the surface of a structural material to be tested and observing an electronic indicator device connected to the probe to display a reading indicative of the transmission of vibrations through the structural material. The usual technique for scanning the probe over the material surface is to make slightly overlapping loop passes.